The present invention relates to improvements in pressure regulating methods and apparatus.
It is often necessary to design a hydraulic control system in such a way that a regulating or control valve (e.g., a pressure reducing valve) is installed downstream of a pilot valve or servo valve (hereinafter called pilot valve). The latter serves to select fluid pressure in a (pilot) conduit which is connected with an inlet of the regulating valve. An outlet of the regulating valve serves to convey fluid to one or more consumers, e.g., to a so-called continuously variable transmission (CVT). Reference may be had, for example, to commonly owned U.S. Pat. No. 5,667,448 (granted Sep. 16, 1997 to Oswald Friedmann for "POWER TRAIN") and to commonly owned U.S. Pat. No. 5,674,155 (granted Oct. 7, 1997 to Dieter Otto et al. for "METHOD OF AND APPARATUS FOR TRANSMITTING TORQUE IN THE POWER TRAINS OF MOTOR VEHICLES"). The disclosures of the just enumerated US patents as well as of all other patents and patent applications (including commonly owned pending German priority patent application Ser. No. 198 29 148.5 filed Jun. 30, 1998) are incorporated herein by reference.
A conventional pilot valve-regulating valve combination can employ a pilot valve having an inlet connected to a source of constant-pressure (or substantially constant-pressure) fluid by a constant-pressure conduit which contains a flow restrictor) e.g., an adjustable diaphragm, and can communicate with a pilot conduit serving to convey fluid to the regulating valve. An adjustable valving element of the pilot valve can divert varying quantities of fluid from the pilot conduit (downstream of the flow restrictor) to a sump or to another destination so that the pilot valve can select the pressure of fluid entering the body of the regulating valve.
A drawback of heretofore known pressure regulating methods and apparatus of the just outlined character is that (for example, due to often unavoidable leakage of fluid from the pilot valve and/or from other parts of the apparatus), the pressure in the pilot conduit (between the pilot valve and the regulating valve) cannot match the pressure of fluid in the constant-pressure conduit. Moreover, even if the valving element of the pilot valve is moved to a fully open position (in which the valving element does not interfere with the flow of fluid from the pilot conduit, through the pilot valve, and on to to the sump), the pilot pressure of fluid (in the pilot conduit) still exceeds zero pressure. Such residual pressure in the pilot conduit is attributable to losses developing in the region of the flow restrictor, e.g., to losses in the path defined by the constant-pressure conduit, by the flow restrictor, by a branch conduit connecting the constant-pressure conduit (downstream of the flow restrictor) to the inlet of the pilot valve, and by the conduit connecting the outlet of the pilot valve with the sump or with another receptacle for hydraulic fluid.
A further drawback of the aforedescribed conventional pressure regulating apparatus is that, due to turbulence which develops in the fluid path in the region of the flow restrictor, at least a high percentage of kinetic energy of the fluid (such as oil) is ultimately converted into heat.